Walk in darkness . . . walk in the light

Light and darkness are different. They are direct opposites. Here John contrasts in absolute terms two diametrically opposed ways of living. A person who purposely walks in darkness and not in the light is not of God. If these claim fellowship with God they are not truthful! As Eaton states:

He puts it in terms of two realms: light and darkness. There is the holy realm of the character of God with its purity and its clear understanding and its joy in God, and there is the realm of darkness which is the exact opposite. One cannot be enjoying the blessing of God while one's life is characterised by the ways of godlessness. John is quite blunt in his reply. Such people are lying!

The Greek verb translated walk suggests that John had in mind regular patterns of life, or consistent lifestyles. Walking in the light does not imply perfection or sinlessness. Everyone sins (1 John 1:8, 10), and even those walking in the light sin when the goals and patterns of their lives stray into paths of darkness. This is why John warned his readers against sin (1 John 2:1). True believers are set free from the dominion and domination of sin, but not from its influence. See "Sanctification: When Will I Stop Sinning?," "Is it possible not to sin? - 1 John 3:9," and "Deliberately keep on sinning - Hebrews 10:26" below.

John refers to fellowship with one another and the blood. As John emphasizes throughout this letter, a right relationship with God through the blood of Christ (see "Union With Christ" below) is closely joined to fellowship with other believers. Those who are forgiven of their sins will live in fellowship and love with others who are forgiven. See WCF 18.3.

walk in darkness… in the light. The OT told God’s people not to mix up right and wrong, or light and darkness (see notes on Isa. 2:5; 5:20). The metaphor of walking has to do with a person’s behavior or pattern of life. This is the clear sense of the word “walk” in 2:9-11, where a person knows they are walking in the light if they love other Christians. This not to say “walking in the light” means a life of sinless perfection. John clearly disallows that meaning in 2:1. Rather, it describes a pattern of life that imitates God’s character (1:5-7), depend on God’s mercy for cleansing from sin (1:7, 9; 2:1-2), obeys God’s commands (2:3-11), and resists the ways of the world (2:15-17), including false teaching (2:18-27). blood… cleanses. See WCF 18.3. This phrase is connected to fellowship, showing how a right relationship with God is connected to right relationships with God’s people. People who have been forgiven will be in fellowship with other forgiven people. In the OT, the blood of a sacrificial animal set people apart as sacred for God by making atonement (see note on Lev. 16:30). The word cleanses here and in v. 9 is about removing the sin which separates a person from God’s presence and purposes.